Recently, more stringent regulations of particulate matter emitted by diesel engines have been passed in Europe and the United States. To meet these regulations, it is expected that particulate filters will be necessary.
These particulate filters will have to meet multiple contradictory exacting requirements. For example, the filter must have sufficient porosity (generally greater than 55 percent porosity) while still retaining most of the emitted micrometer sized diesel particulates (generally greater than 90 percent capture of the emitted particulates). The filter must also be permeable enough so that excessive back pressure does not occur too quickly, while still being able to be loaded with a great amount of soot before being regenerated. The filter must withstand the corrosive exhaust environment for long periods of time. The filter must have an initial strength to be placed into a container attached to the exhaust system. The filter must be able to withstand thermal cycling (i.e., retain adequate strength) from the burning off of the soot entrapped in the filter (regeneration) over thousands of cycles where local temperatures may reach as high as 1600° C. From these stringent criteria, ceramic filters have been the choice of material to develop a diesel particulate filter.
Early on, ceramic filters of sintered cordierite were explored as a possible diesel particulate filter. Cordierite was explored because of its low cost and use as a three-way catalyst support in automotive exhaust systems. Unfortunately, cordierite has not displayed the capability of high porosity, high permeability and high soot loading in conjunction with retained strength after thousands of regeneration cycles to satisfy the stringent requirements described above.
More recently, silicon carbide has become of interest as filters because of its high strength and ability to maintain strength after thermal cycling. However, silicon carbide suffers, for example, from having to be sintered at high temperature using expensive fine silicon carbide powder. Because silicon carbide is sintered, the pore structure that develops results in limited soot loading before excessive back pressure develops.
In addition, mullite of interlaced crystals grown together have been described by U.S. Pat. No. 5,098,455, for use as a diesel particulate trap. These mullite filters have not met the demanding criteria of a diesel filter such as thermal shock resistance.
Accordingly, it would be desirable to provide both a formation method and a ceramic material that solves one or more of the problems of the prior art, such as one of those described above.